1. Field of Invention
The present invention relates to a process for the preparation of cellular or noncellular plastics. More particularly, the invention relates to a process for the preparation of polyurethane coatings by reacting at least two components with each other, by feeding the components into a mixing zone separately, and mixing them together, then discharging the reaction mixture from the mixing zone while adding air heated to a temperature of 100.degree. C. to 1200.degree. C.
2. Description of Prior Art
In known processes for the preparation of cellular or noncellular plastics, the reaction components are usually injected into a mixing chamber from opposing directions. An individual nozzle must be present for each component. Disadvantages of this process result during mixing if one of the components must be injected at relatively low volumes compared to the others, or if there are great differences between the various injection pressures.
It is known that the ability of multi-component plastics to mix well is increased through the use of finely dispersed air. In order to accomplish this, air can be fed directly into the mixing chamber or fed into the component supply vessels and mixed there with the respective components. If air is added to the components in the supply reservoirs, mixing occurs very quickly, so that this method of adding air is largely ineffective. Adding air directly into the mixing chamber is also not completely satisfactory, since this frequently causes uneven or atomized flow out of the mixing chamber. This is a disadvantage when pouring must be accurate or in particular when vertical surfaces must be coated.
The coating of molds with polyurethane is also known from numerous publications. This may be accomplished, for example, by electrostatically applying hot-melt polyurethane elastomer powder to the inside surface of the mold, or heating the molding tool to the melting point of the elastomer in order to obtain a polyurethane layer. In a different method, sheets of polyurethane are placed in the mold and, if necessary, held on the surface of the mold by means of vacuum. The reactive polyurethane mixture is fed into the thus prepared mold and is allowed to cure in the closed mold. Processes are also known in which the inside surface of the mold is sprayed with a two-component polyurethane formulation. Here it takes a few minutes until a cured film has formed. A disadvantage here is that the polyurethane mixture can flow off on vertical surfaces, sharp corners, or chamfered edges--provided that outside force is not involved--before the mixture has cured sufficiently. Similar problems are encountered when the polyurethane mixture is diluted with inert solvents in order to achieve a low-viscosity, sprayable mixture or when the undiluted polyurethane mixture is applied by means of high-pressure airless spraying.
In order to avoid these disadvantages, it was proposed that agents be incorporated in the polyurethane mixtures to increase thixotropy--for example, silicone dioxide, polyamides, or polyacrylamides. However, this can adversely affect film formation, causing porous or streaked surfaces. In Federal Republic of Germany Offenlegungsschrift No. 29 32 315 (U.S. Pat. No. 4,294,880) primary aliphatic diamines are incorporated in the polyurethane composition in order to increase thixotropy. A lightfast polyurethane outer skin having a thickness of at least 0.5 mm, which cures relatively quickly, is obtained in accordance with Federal Republic of Germany OS No. 30 14 161 (U.S. Pat. No. 4,389,454) through the reaction of aliphatic polyisocyanates with a mixture of relatively high molecular weight polyhydroxyl compounds, low molecular weight chain extenders, and aliphatic and/or cycloaliphatic primary or secondary diamines in the presence of preferably synergistically acting combinations of catalysts. This last process produces good lightfast polyurethane coatings, but it is sensitive and requires high-quality processing.
Essentially two processes are available for spraying two-component polyurethane systems. In the so-called airless process, the initial components are mixed by means of a high pressure, counterflow process or in a static mixing tube and are then atomized by high pressure nozzles. In the so-called air-injection process, the initial components are mixed in static mixing tubes with a low pressure process or they are mixed in a mixing chamber using mechanical agitators. The component mixture is combined with air when it is discharged from the nozzle, which atomizes it. Since the time which the component mixture resides in the mixing chamber is relatively long, the pot time of the reaction mixture must be adjusted to be correspondingly long in order to prevent partial reaction of the mixture in the mixing chamber.
In order to achieve optimal mixing in the mixing chamber as well as good nozzle spraying, the reaction mixture must be adjusted to have as low a viscosity as possible, and preferably must possess a viscosity less than 400 m.Pa.s. A limited reduction in viscosity can be achieved in conventional processes by heating the initial components, preferably to temperatures from 30.degree. to 80.degree. C. However, increasing the temperatures significantly reduces the pot time of the reaction mixture. In order to prevent a buildup of material on the mixing equipment and nozzles, the hot, liquid reaction mixtures must not exhibit any increase in viscosity as a result of the reaction from the point they are mixed until they leave the nozzle. Moreover, they must have a pot time of at least 30 seconds.
Alternatively, the reaction mixture can be mixed with air when it leaves the mixing chamber, so that it is sprayed in the form of droplets. The addition of air cools the reaction mixture and lengthens pot time. A reaction mixture, for example, a two-component polyurethane system, sprayed onto vertical walls of molds as described above, results in a slow reaction at room temperature for layer thicknesses equal or greater to 0.3 mm which causes the reaction mixture to flow off the mold walls in an undesirable manner. In particular, "runs" form prior to the start of the curing phase on vertical surfaces, recesses, or sharp edges. If this sheeting or formation of "runs" in the layer of sprayed on reaction mixture is to be prevented, several relatively thin layers must be sprayed on separately and dried between sprayings. Such a process is labor-intensive and therefore expensive.